Cathode material with oxygen vacancy and manufacturing process thereof

ABSTRACT

A cathode material with oxygen vacancy is provided. The cathode material includes a lithium metal phosphate compound having a general formula LiMPO 4-Z , wherein M represents at least one of a first-row transition metal, and 0.001≦z≦0.05.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of U.S. patentapplication Ser. No. 14/367,112 filed on Jun. 19, 2014, which is a USnational phase application of international application No.PCT/CN2012/087171 filed on Dec. 21, 2012, which claims the benefit ofU.S. Provisional Application No. 61/578,329 filed on Dec. 21, 2011. Eachof the aforementioned patent applications is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a cathode material, and moreparticularly to a cathode material with oxygen vacancy.

BACKGROUND OF THE INVENTION

With the diversified development of electronic products, the demands onportable energy sources are gradually increased. For example, consumerelectronic devices, medical instruments, electric bicycles, electricvehicles or electric hand tools use portable power sources as sources ofelectric power. Among these portable power sources, rechargeablebatteries (also referred as secondary cells) are widely used because theelectrochemical reactions thereof are electrically reversible. Moreover,among the conventional secondary cells, lithium-ion secondary cells havehigh volumetric capacitance, low pollution, good charge and dischargecycle characteristics, and no memory effect. Consequently, thelithium-ion secondary cells are more potential for development.

As known, the performance of the secondary cell is influenced by manyfactors. Generally, the material for producing a positive electrode(also referred as a cathode) is more critical to the performance of thesecondary cell. Because of good electrochemical characteristics, lowenvironmental pollution, better security, abundant raw material sources,high specific capacity, good cycle performance, good thermal stabilityand high charge/discharge efficiency, the lithium iron phosphate-basedcompound having an olivine structure or a NASICON structure isconsidered to be the potential lithium-ion battery cathode material.

However, due to the hindrance of the crystalline structure, the lithiumiron phosphate compound has very low electronic conductivity and lowlithium diffusion rate. Consequently, the applications of the lithiumiron phosphate compound are restricted. Therefore, it is an importantissue to enhance the electrical performance of the lithium ironphosphate compound.

SUMMARY OF THE INVENTION

The present invention provides a cathode material with oxygen vacancy.During the process of preparing the lithium metal phosphate compound, aportion of the phosphate is substituted by an anionic group with three(or less) oxygen atoms. Consequently, a lithium metal phosphate compoundwith oxygen vacancy is produced. Moreover, the conductive performance ofthe cathode material is enhanced, and the electric capacity of thecathode material is increased.

In accordance with an aspect of the present invention, there is provideda cathode material with oxygen vacancy. The cathode material includes alithium metal phosphate compound having a general formula LiMPO_(4-Z),wherein M represents at least one of a first-row transition metal, and0.001≦z≦0.05.

In accordance with another aspect of the present invention, there isprovided a process of manufacturing a cathode material with oxygenvacancy. Firstly, a lithium metal phosphate raw material is provided.The lithium metal phosphate raw material is a mixture of alithium-containing first material, a metal-containing second materialand a phosphate-containing third material, wherein 0.1˜5 mol % ofphosphate in the third material is substituted by an anionic group [XO₃^(n−)]. Then, the first material, the second material and the thirdmaterial carry out a dry processing reaction or a wet processingreaction. Afterwards, the first material, the second material and thethird material are thermally treated by sintering. Consequently, alithium metal phosphate compound with oxygen vacancy is produced.

The above contents of the present invention will become more readilyapparent to those ordinarily skilled in the art after reviewing thefollowing detailed description and accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the secondary particle diameter change of the slurry withdifferent phosphite substitution ratios;

FIG. 2 schematically illustrates the SEM photograph of the productpowder with 0.5% phosphite substitution ratio;

FIG. 3 schematically illustrates the NPD patterns of some product powdersamples with different phosphite substitution ratios; and

FIG. 4 schematically illustrates a diffraction peak pattern contributedby O atoms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically withreference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this invention arepresented herein for purpose of illustration and description only. It isnot intended to be exhaustive or to be limited to the precise formdisclosed.

The present invention provides a cathode material. First of all, anorganic polymeric chelating agent is provided. The chelating end oforganic polymeric chelating agent has an anionic group [XO₃ ^(n−)],wherein X=P, S, N, and 1≦n≦3. By carrying out a dispersion process andadjusting the molar ratio of [XO₃ ^(n−)]/PO₄ ³⁻, a lithium metalphosphate compound with oxygen vacancy is produced. The lithium metalphosphate compound has a general formula LiMPO_(4-Z), wherein Mrepresents at least one of a first-row transition metal, and0.001≦z≦0.05. Besides, since the (PO₄) in the lithium metal phosphatecompound of the present invention is substituted by the anion group [XO₃^(n−)], the general formula con also be written asLiM(PO₄)_(1-z)(XO₃)_(z), and for example, the anionic group (XO₃) inthis general formula represents PO₃ ²⁻, SO₃ ²⁻, or NO₃ ⁻.

In particular, the anionic group [XO₃ ^(n−)] only contains three oxygenatoms. During the synthesis process of the lithium metal phosphatecompound, a portion of the phosphate with four oxygen atoms issubstituted by the anionic group [XO₃ ^(n−)]. Consequently, the producedlithium metal phosphate compound has oxygen vacancy. Since this type oflithium metal phosphate compound has oxygen vacancy, the spatialstructure of the unit cell is changed. Under this circumstance, thelithium diffusion rate is increased, the conductive performance of thecathode material is enhanced, and the electric capacity of the cathodematerial is increased.

The present invention further provides a process of manufacturing acathode material with oxygen vacancy. Firstly, a lithium metal phosphateraw material is provided. The lithium metal phosphate raw material is amixture of a lithium-containing first material, a metal-containingsecond material and a phosphate-containing third material. In the thirdmaterial, 0.1˜5 mol % of phosphate is substituted by the anionic group[XO₃ ^(n−)]. After the first material, the second material and the thirdmaterial are subject to a dry processing reaction or a wet processingreaction, the mixture is thermally treated by sintering. Consequently,lithium metal phosphate compound with oxygen vacancy is produced. Thelithium metal phosphate compound has a general formula LiMPO_(4-Z). Inthe general formula, M represents at least one of a first-row transitionmetal selected from iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni),titanium or chromium (Cr), 0.001≦z≦0.05, X═P, S, N, and 1≦n≦3. Forexample, the anionic group [XO₃ ^(n−)] represents PO₃ ³⁻, SO₃ ²⁻ or NO₃⁻.

In an embodiment, the first material includes but is not limited tolithium hydroxide or lithium carbonate; the second material includes butis not limited to iron powder, ferric oxalate or ferrous chloride; andthe third material contains a phosphate source (e.g. phosphoric acid)and a phosphite organic compound for substituting the phosphate (e.g.phosphite ester or organophosphite). An example of the phosphite organiccompound includes but is not limited toisopropyl-idene-diphenol-phosphite ester resin.

Alternatively, in another embodiment, the third material contains aphosphate source (e.g. phosphoric acid) and a hydrophosphite organiccompound for substituting the phosphate (e.g. tert-butyl glycinatehydrophosphite, distearyl hydrophosphite or diethyl hydrophosphite).

Optionally, during the process of manufacturing the cathode material, afourth material such as a metal oxide may be added to the raw material.For example, the metal oxide includes but is not limited to MgO, TiO₂ orV₂O₅.

The preparation process and efficacy of the cathode material with oxygenvacancy will be illustrated in the following examples.

EXAMPLE 1

Iron powder (2740 g) was added to phosphoric acid solution (5734 g,85%˜85.5%, Mw: 97.97), and a reaction of the mixture was carried out for24 hours. After the reaction was completed, a phosphite organiccompound, isopropyl-idene-diphenol-phosphite ester resin (600 g, Mw:2400), was added to the reaction product. In addition, fructose (625 g)and V₂O₅ (<1 wt %) were added, and the mixture was ground and dispersed(grinding speed: 550 rpm˜650 rpm, zirconia ball: 0.5 mm˜1.0 mm). Then, alithium source (e.g. LiOH, Li/P molar ratio: 0.990˜4.005) was added toform an aqueous solution (solid content: 25%˜45%). Then, the aqueoussolution was spray-dried (hot-air temperature: 200˜220° C., outlettemperature: 85˜95° C.) to form a powdery mixture. Under a protectiveatmosphere such as nitrogen or argon gas, the powdery mixture wassintered (<650° C., crucible capacity: 60%˜80%). After the sinteringprocess was completed, a product powder was obtained.

The proportion of phosphite for substituting phosphate was in the rangebetween 1 and 5 mol %. Since the phosphate content was about 49.75moles, the phosphite content was about 0.25 mole. That is, 0.5% ofphosphate was substituted by phosphite. The product powder is a lithiumiron phosphate compound having a general formula LiMPO_(4-Z), wherein zis the substitution ratio (0.005). The oxygen number is 3.995 (i.e.4×(1−0.005)+3×0.005=4−0.005=3.995). In other words, the chemical formulaof the lithium iron phosphate compound is LiFePO_(3.995).

EXAMPLES 2-6

Except for the amount of the phosphoric acid and the phosphite organiccompound, the manufacturing processes of Examples 2˜6 were substantiallyidentical to the manufacturing process of Example 1. Consequently,various products with different phosphite substitution ratios wereprepared. In Examples 2˜6, the phosphite substitution ratios are 0.1%,0.3%, 0.75%, 2% and 5%, respectively.

EXAMPLE 7

Except that the phosphite organic compound was replaced by ahydrophosphite organic compound, the manufacturing process of Example 7was substantially identical to the manufacturing process of Example 1.For example, the hydrophosphite organic compound includes but is notlimited to tert-butyl glycinate hydrophosphite, distearyl hydrophosphiteor diethyl hydrophosphite.

EXAMPLE 8

In comparison with the wet process of Example 1, this embodiment used adry process to prepare the lithium iron phosphate compound with oxygenvacancy. A solid mixture of lithium carbonate, ferric oxalate andammonium hydrogen phosphate at a molar ratio 0.995˜4.005:0.985˜0.995:1was prepared. Then, the mixture was sintered at the temperature 325°C.±25° C. The sintering process has a dehydrating function and acarbonate removal function. After the sintering process was completed, aphosphite organic compound (e.g. phosphite ester or organophosphite) wasadded to the precursor. The proportion of phosphite for substitutingphosphate was in the range between 1 and 5 mol %. Then, a small amountof organic solution was added to prepare slurry with a solidcontent >80%. Under a protective atmosphere such as nitrogen or argongas, the powdery mixture was sintered (<700° C.). After the sinteringprocess was completed, a product powder was obtained.

EXAMPLE 9

The following table 1 shows the secondary particle diameter change ofthe slurry with different phosphite substitution ratios. From the D₅₀and D₇₀ values, the slurry with phosphite to substitute phosphate hasmuch smaller second particle diameter (μm) than the slurry withoutphosphite substitution. The result shows that the addition of thephosphite organic compound is effective to increase the grindingefficiency. Consequently, the particle dispersion efficacy is enhanced,the aggregation in the sintering process is reduced, and the particlesize of the product powder is smaller.

TABLE 1 D₅₀ D₇₀ No substitution 1.05 1.3028 0.5% substitution 0.350.7025 0.3% substitution 0.39 0.7305 0.1% substitution 0.45 0.8104

The data about the particle diameter listed in Table 1 may be plotted asFIG. 1. According to the positive linear relationship, the addition ofthe phosphite organic compound is effective to increase the grindingefficiency.

Although the particle diameter of the slurry is effectively reduced, theaddition of the lithium source (e.g. LiOH) increases the reaction rateand thus the slurry suddenly becomes sticky. For solving this drawback,the following measures may be adopted: (A) the grinding speed isdecreased and the grinding efficiency in unit time is decreased, (B) thepH range while adding LiOH is properly controlled, (C) the temperaturewhile adding LiOH is effectively controlled, and (D), the particlediameter of the white slurry before LiOH addition is properly controlledand the operating range is defined.

EXAMPLE 10

The following table 2 shows the secondary particle diameter change ofthe slurry with a 0.5% phosphite substitution ratio. In Table 2, S0represents the product powder with no phosphite substitution, and S1 andS2 represent two product powder samples with 0.5% phosphite substitutionratio. From the D₅₀ and D₉₅ values, the product powder with phosphite tosubstitute phosphate has much smaller second particle diameter (μm) thanthe product powder without phosphite substitution.

TABLE 2 D₅₀ D₉₅ S0 28.4 57.68 S1 15.69 47.46 S2 13.91 45.93

The SEM photograph of the S2 product powder is shown in FIG. 2. From theSEM photograph, the primary particle diameter is also reduced. It ispresumed that the oxygen vacancy may result in lattice defects.Consequently, after phase formation, the growth of the crystal particlewill be inhibited. Under this circumstance, the primary particlediameter is smaller, the C rate is better, and the low temperatureperformance is enhanced.

EXAMPLE 11

The physical data of some product powder samples with 0.5% phosphitesubstitution ratio are shown in Table 3 as follows. In Table 3, S3˜S9represent different product powder samples (50 moles for verification).It is found that the surface areas of the product powder samples areeffectively increased. That is, the product powder samples have morepores, and the primary particle diameters are smaller.

TABLE 3 Surface ICP Sample Density D₁₀ D₅₀ D₉₅ Area Li Fe P STD S3 0.552.56 10.35 24.93 15.23 1.056 0.955 1 0.999 0.987 1 S4 0.65 2.68 24.0546.27 18.70 0.988 0.982 1 0.983 0.982 1 S5 0.56 2.71 15.75 43.7 20.520.994 0.974 1 1.003 0.981 1 S6 0.76 2.60 20.75 48.56 18.25 1.001 0.972 11.003 0.981 1 S7 0.62 3.20 14.43 44.25 17.26 1.023 0.965 1 0.995 0.972 1S8 0.85 1.36 18.14 52.08 17.31 0.977 0.961 1 0.973 0.962 1 S9 0.851 1.7716.72 46.16 18.05 0.953 0.946 1 0.952 0.946 1

EXAMPLE 12

The electrical data of some product powder samples with 0.5% phosphitesubstitution ratio are shown in Table 4 and Table 5 as follows. In Table3, S2˜S9 represent different product powder samples (small quantity of 2moles for verification in Table 4 and large quantity of 50 moles forverification in Table 5). From these two tables, it is found that theelectric capacity values of the product powder samples at a dischargerate of 2C are all larger than or close to 140 mAh/g. Moreover, thebehaviors of the product powder sample S9 at higher charge/dischargerates are also observed. The electric capacity value of the productpowder sample S9 at a discharge rate of 2C is larger than 140 mAh/g.

TABLE 4 Capacity Sample 0.1C-C 0.1C-D 0.1C-C 0.1C-D 1C-C 2C-D 1C-C 2C-DS2 163.99 157.69 157.75 156.97 157.46 140.35 140.91 139.65

TABLE 5 Sample Capacity 0.1C-C 0.1C-D 0.1C-C 0.1C-D 1C-C 2C-D 1C-C 2C-DS3 166.63 155.68 158.39 156.17 157.00 142.42 142.28 142.23 S4 171.83154.69 170.56 153.51 157.75 137.76 135.93 137.07 S5 162.93 155.63 156.94155.28 156.57 137.65 138.82 137.56 S6 162.90 157.55 158.16 157.96 155.92140.87 137.82 140.73 S7 164.86 158.66 159.86 159.63 157.80 146.46 143.11145.92 S8 175.09 154.72 162.16 157.37 161.36 136.55 136.38 137.41 0.1C-C0.1C-D 0.2C-C 0.5C-D 2C-C 2C-D 2C-C 2C-D S9 165.77 152.60 157.38 152.28151.36 145.87 146.25 146.33

From the above experiments, it is found that the oxygen vacancy mayresult in the spatial structure change of the unit cell. Under thiscircumstance, the lithium diffusion rate is increased, the conductiveperformance of the cathode material is enhanced, and the electriccapacity of the cathode material is increased.

EXAMPLE 13

The neutron powder diffraction (NPD) patterns of some product powdersamples with different phosphite substitution ratios are shown in FIGS.3 and 4. Since X-ray is not suitable to observe Li but neutron is moresensitive to light element than X-ray, the neutron powder diffractionpattern may be used to observe the presence of Li atom. Moreover, due tothe difference between the O-scattering cross sections, the diffractionpeak change can be obviously observed.

FIG. 3 schematically illustrates the NPD patterns of some product powdersamples with different phosphite substitution ratios. From the NPDpatterns, the phase of LiFePO₄ is measured. Sine P is only bonded to O,none of the elements of LiFePO₄ can be substituted. In other words, thecharge balance problem and the energy balance problem are no longergenerated. The bonding between P, Fe and O plays an important role inproviding more channels for allowing free access of lithium ion.Moreover, due to the ordered lattice arrangement, the structure is morestable. Consequently, the lithium ion can move in or move out moresmoothly.

FIG. 4 schematically illustrates a diffraction peak pattern contributedby O atoms. As the phosphite substitution ratio increases from 0% to 2%,the NPD patterns show that the lattice arrangement is better and theoxygen vacancy is increased. Although the oxygen vacancy is increased byabout 1.3%, more lithium ions can move out the crystalline structure.Since the oxygen vacancy is a main path of moving in/out the lithiumion, the possibility of generating the dead lithium ion is largelyreduced. This is the main reason why the electric capacity is increased.However, if the phosphite substitution ratio is 5%, the crystal latticedistortion becomes more serious. Under this circumstance, the electriccapacity is impaired.

From the above descriptions, the present invention provides a cathodematerial with oxygen vacancy. The cathode material comprises a lithiummetal phosphate compound having a general formula LiMPO_(4-Z), wherein Mrepresents at least one of a first-row transition metal, and0.001≦z≦0.05. During the process of preparing the lithium metalphosphate compound, a portion of the phosphate is substituted by ananionic group with three (or less) oxygen atoms. Consequently, a lithiummetal phosphate compound with oxygen vacancy is produced. Since thistype of lithium metal phosphate compound has oxygen vacancy, the spatialstructure of the unit cell is changed. Under this circumstance, thelithium diffusion rate is increased, the conductive performance of thecathode material is enhanced, and the electric capacity of the cathodematerial is increased.

While the invention has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention needs not be limited to the disclosedembodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

What is claimed is:
 1. A cathode material with oxygen vacancy, thecathode material comprising a lithium metal phosphate compound having ageneral formula LiM(PO₄)_(1-z)(XO₃)_(z),wherein M represents at leastone of a first-row transition metal, the anionic group (XO₃) representsPO₃ ³⁻, SO₃ ²⁻ or NO₃ ⁻, and 0.001≦z≦0.05.
 2. The cathode materialaccording to claim 1, wherein M is iron (Fe), manganese (Mn), cobalt(Co), nickel (Ni), titanium or chromium (Cr).
 3. The cathode materialaccording to claim 1, wherein M is iron (Fe).